The present invention relates generally to electrical protection circuitry for magnetic disk drives and more particularly to methods and apparatus using fusible-links to protect against electrostatic discharge in read/write transducers utilizing magnetoresistive read sensors.
Magnetic head disk drive systems have been widely accepted in the computer industry as a cost effective form of data storage. In a magnetic disk drive system a magnetic recording medium, in the form of a disk, rotates at high speed while a magnetic read/write transducer, referred to as a magnetic head, "flies" slightly above the surface of the rotating disk. The magnetic disk is rotated by means of a spindle drive motor. The magnetic head is attached to, or formed integrally with, a "slider" which is suspended over the disk by a spring-loaded suspension attached to a support arm known as the actuator arm. As the magnetic disk rotates at operating speed, the moving air generated by the rotating disk in conjunction with the physical design of the slider operate to lift the magnetic head allowing it to glide or fly slightly above and over the disk surface on a cushion of air, referred to as an air bearing. The flying height of the magnetic head over the disk surface is typically only a few microinches or less and is primarily a function of disk rotation speed, the aerodynamic properties of the slider assembly and the force exerted by the spring-loaded actuator arm.
Thin film heads are particularly susceptible to damage from electrostatic discharge. A major problem that is encountered during manufacture, handling and use of magnetic recording transducers, referred to as heads, is the buildup of electrostatic charges on the various elements of a head or other objects which come into contact with the heads and the accompanying spurious discharge of the static electricity thus generated. For example, static charges may be produced by the presence of certain materials, such as plastics, during manufacture and subsequent handling of the heads. These static charges arc across the edge of the insulating layer between the magnetic pole tips and adjacent conductive layers which are exposed and positioned adjacent to the transducing gap at the air bearing surface facing the recording medium thus causing erosion of the pole tips and degradation of the transducer in reading and writing of data.
The above-described electrostatic discharge (ESD) problems associated with thin film inductive read/write heads are well-documented and several solutions have been proposed. Commonly assigned U.S. Pat. No. 4,317,149 issued to Elser et al discloses an inductive head having short discharge paths formed by the deposition of conductive material in the recesses formed in an insulating layer so that the static electric discharge will occur in areas displaced from the critical pole tip and gap area at the slider air bearing surface. U.S. Pat. No. 4,800,454 issued to Schwarz et al discloses an inductive head assembly wherein the magnetic pole piece and the inductive winding is coupled to the slider body to allow discharge of any static electric charges which may build up. A diode, with high forward and reverse voltage drops, or a fusible-link connects the slider body to the inductive winding. Schwarz et al also teach a method of opening the fusible-link by applying current between a signal lead and the slider body.
Magnetoresistive (MR) sensors are well known and are particularly useful as read elements in magnetic transducers, especially at high recording densities. The MR read sensor provides a higher output signal than an inductive read head. This higher output signal results in a higher signal to noise ratio for the recording channel and thus allows greater areal density of recorded data on a magnetic disk surface to be achieved. As described above, when an MR sensor is exposed to ESD, or even a voltage or current input larger than that intended under normal operating conditions, referred to as electrical overstress or EOS, the MR read sensor and other parts of the head may be damaged. This sensitivity to electrical damage is particularly severe for MR read sensors because of these sensors' relatively small physical size. For example, an MR sensor used for extremely high recording densities will have a cross-section of 100 Angstroms (.ANG.) by 1.0 micrometers (um) or smaller. Accidental discharge of voltages of only a few volts through such a physically small resistor is sufficient to produce currents capable of severely damaging or completely destroying the MR sensor. The nature of the damage which may be experienced by an MR sensor varies significantly, including complete destruction of the sensor via melting and evaporation, contamination of the air bearing surface, generation of shorts via electrical breakdown, and milder forms of damage in which the head performance may be degraded. This type of damage has been found to occur during both processing and use and poses a serious problem in the manufacturing and handling of magnetic heads incorporating MR read sensors.